[OP13-001_p1] - Monkey.D.Luffy (Carrying On His Will: Alternate Art)

My opponent attacked this Leader when I had 6 active DON!! cards; I used a 1-cost Event with [Counter], and the number of my active DON!! cards became 5. Can I then use this [On Your Opponent's Attack] effect to give my Leader +2000 power?

No, you cannot. The [On Your Opponent's Attack] effect would activate before you used [Counter].